The present invention relates generally to implantable medical devices and more particularly to controlling surface properties of implantable biocompatible materials suitable for fabrication of implantable medical devices. Implantable medical devices are fabricated of materials that are sub-optimal in terms of the biological response they elicit in vivo. Many conventional materials used to fabricate implantable devices, such as titanium, polytetrafluoroethylene, silicone, carbon fiber and polyester, are used because of their strength and physiologically inert characteristics. However, tissue integration onto these materials is typically slow and inadequate. Certain materials, such as silicone and polyester, elicit a significant inflammatory, foreign body response that drives fibrous encapsulation of the synthetic material. The fibrous encapsulation may have significant adverse effects on the implant. Moreover, conventional biomaterials have proved inadequate in eliciting a sufficient healing response necessary for complete device integration into the body. For example, in devices that contact blood, such as stents and vascular grafts, attempts to modify such devices to promote endothelial cell adhesion may have a concomitant effect of making the devices more thrombogenic.
When implanted, conventional blood-contacting implantable devices, such as stents, stent-grafts, grafts, valves, shunts and patches, fail to develop a complete endothelial layer, thereby exposing the device material to thrombus formation or smooth muscle cell proliferation, and ultimate failure of the implanted device. It has been recognized that, when implanted into the body, metals are generally considered to have superior biocompatibility than polymers used to fabricate commercially available polymeric grafts.
In investigating cellular interactions with prosthetic material surfaces, it has been found that cell adhesion to the material surface is mediated by integrins present on cell membranes that interact with the prosthetic surface. Integrins are the most prominent member of a class of extracellular matrix (ECM) adhesion receptors. Integrins are a large family of heterodimeric transmembrane proteins with different α and β subunits. Integrins are regulated at several levels. Modulation of the affinity of the adhesion receptor for ligand, termed affinity modulation, is a mechanism for activation of platelet aggregation and is believed to underlie activation of leukocyte adhesion. Adhesive strengthening by clustering of adhesion receptors or by cytoskeletal-dependent processes such as cell spreading has been shown to be crucial for strong cellular attachment, control of cell growth and cell motility. Under high shear forces present in flowing blood, leukocytes first tether, then roll along the vessel surface. When a local signal, e.g., a cytokine, is released in their vicinity, the leukocyte arrests, develops a firm adhesion then migrates across the endothelium. Tethering, rolling, arrest and adhesion tightening are all known to result from activation of leukocyte integrins.
Once adhered to a surface, cell spreading and migration are associated with assembly of focal adhesion junctions. Cell migration entails the coordination of cytoskeletal-mediated process extension, i.e., filopodia and lamellopodia, formation of adhesive contacts at the leading edge of a cell, breaking adhesive contacts, and cytoskeletal retraction at the trailing edge of the cell. Focal adhesions are comprised of integrins as the major adhesion receptors along with associated cytoplasmic plaque proteins. Assembly of focal adhesions is regulated by extracellular ligand binding events and by intracellular signaling events. Ligand binding controls localization of β1- and β3-containing integrins into focal adhesions. The cytoplasmic domains of the β subunits have intrinsic signals for focal adhesion localization, but incorporation of the integrins into focal adhesions is prevented by the a subunits of the heterodimers. Ligand binding, however, relieves this inhibition and allows the subunit cytoplasmic tail signals to recruit the integrin dimmer into the focal adhesion.
Attempts at coating implanted metal devices, such as stents, with proteins that contain the Arg-Gly-Asp (RGD) attachment site have been made with some success. The RGD sequence is the cell attachment site of a large number of adhesive extracellular matrix, blood, and cell surface proteins and many of the known integrins recognize the RGD sequence in their adhesion protein ligands. Integrin-binding activity may also be reproduced by synthetic peptides containing the RGD sequence. However, bare metal implanted materials will not, of course, have native RGD attachment sites. Thus, metal implantable devices, such as stents, have been derivitized with polymers having RGD attachment sites bound to the polymer matrix.
It has been found that when prosthetic materials are implanted, integrin receptors on cell surfaces interact with the prosthetic surface. When cells come into contact with the extracellular matrix, such as a prosthetic surface, their usual response is to extend filopodia, and integrins at the tip of the filopodia bind to the extracellular matrix and initiate the formation of focal adhesions. Actin-rich lamellipodia are generated, often between filopodia, as the cell spreads on the extracellular matrix. Fully developed focal adhesions and associated actin stress fibers ensue. These same evens occur during cell migration as cells extend lamellipodia and form focal adhesions to derive the traction necessary for movement. Giancotti, F. G., et al. Science, 285:13 Aug. 1999, 1028-1032.
The integrin receptors are specific for certain ligands in vivo. If a specific protein is adsorbed on a prosthetic surface and the ligand exposed, cellular binding to the prosthetic surface may occur by integrin-ligand docking. It has also been observed that proteins bind to metals in a more permanent fashion than they do to polymers, thereby providing a more stable adhesive surface. The conformation of proteins coupled to surfaces of most medical metals and alloys appears to expose greater numbers of ligands and attract endothelial cells having surface integrin clusters to the metal or alloy surface, preferentially over leukocytes.
Because of their greater adhesive surface profiles, metals are also susceptible to short-term platelet activity and/or thrombogenicity. These deleterious properties may be offset by administration of pharmacologically active antithrombogenic agents in routine use today. Surface thrombogenicity usually disappears 1-3 weeks after initial exposure. Antithrombotic coverage is routinely provided during this period of time for coronary stenting. In non-vascular applications such as musculoskeletal and dental, metals have also greater tissue compatibility than polymers because of similar molecular considerations. The best article to demonstrate the fact that all polymers are inferior to metals is van der Giessen, W J. et al. Marked inflammatory sequelae to implantation of biodegradable and non-biodegradable polymers in porcine coronary arteries, Circulation, 1996:94(7):1690-7.
Normally, endothelial cells (EC) migrate and proliferate to cover denuded areas until confluence is achieved. Migration, quantitatively more important than proliferation, proceeds under normal blood flow roughly at a rate of 25 μm/hr or about 2.5 times the diameter of an EC, which is nominally 10 μm. EC migrate by a rolling motion of the cell membrane, coordinated by a complex system of intracellular filaments attached to clusters of cell membrane integrin receptors, specifically focal contact points. The integrins within the focal contact sites are expressed according to complex signaling mechanisms and eventually couple to specific amino acid sequences in substrate adhesion molecules. An EC has roughly 16-22% of its cell surface represented by integrin clusters. Davies, P. F., Robotewskyi A., Griem M. L. Endothelial cell adhesion in real time. J. Clin. Invest.1993; 91:2640-2652, Davies, P. F., Robotewski, A., Griem, M. L., Qualitiative studies of endothelial cell adhesion, J. Clin. Invest.1994; 93:2031-2038. This is a dynamic process, which involves more than 50% remodeling in 30 minutes. The focal adhesion contacts vary in size and distribution, but 80% of them measure less than 6 μm2, with the majority of them being about 1 μm2, and tend to elongate in the direction of flow and concentrate at leading edges of the cell. Although the process of recognition and signaling to determine specific attachment receptor response to attachment sites is not completely understood, availability of attachment sites will favorably influence attachment and migration. It is known that materials commonly used as medical grafts, such as polymers, do not become covered with EC and therefore do not heal after they are placed in the arteries. It is therefore an object of this invention to replace polymer grafts with metal grafts that can potentially become covered with EC and can heal completely. Furthermore, heterogeneities of materials in contact with blood flow are preferably controlled by using vacuum deposited materials.
There have been numerous attempts to increase endothelialization of implanted medical devices such as stents, including covering the stent with a polymeric material (U.S. Pat. No. 5,897,911), imparting a diamond-like carbon coating onto the stent (U.S. Pat. No. 5,725,573), covalently binding hydrophobic moieties to a heparin molecule (U.S. Pat. No. 5,955,588), coating a stent with a layer of blue to black zirconium oxide or zirconium nitride (U.S. Pat. No. 5,649,951), coating a stent with a layer of turbostratic carbon (U.S. Pat. No. 5,387,247), coating the tissue-contacting surface of a stent with a thin layer of a Group VB metal (U.S. Pat. No. 5,607,463), imparting a porous coating of titanium or of a titanium alloy, such as Ti—Nb—Zr alloy, onto the surface of a stent (U.S. Pat. No. 5,690,670), coating the stent, under ultrasonic conditions, with a synthetic or biological, active or inactive agent, such as heparin, endothelium derived growth factor, vascular growth factors, silicone, polyurethane, or polytetrafluoroethylene (U.S. Pat. No. 5,891,507), coating a stent with a silane compound with vinyl functionality, then forming a graft polymer by polymerization with the vinyl groups of the silane compound (U.S. Pat. No. 5,782,908), grafting monomers, oligomers or polymers onto the surface of a stent using infrared radiation, microwave radiation or high voltage polymerization to impart the property of the monomer, oligomer or polymer to the stent (U.S. Pat. No. 5,932,299). However, all these approaches do not address the lack of endothelialization of polymer grafts.
In accordance with the present invention, the capacity for complete endothelialization of conventional implantable materials, including metals and polymers, may be enhanced by imparting a pattern of chemically and/or physiochemically active features onto a blood contacting surface of the implantable material. The inventive implantable metal devices may be fabricated of polymers, pre-existing conventional wrought metallic materials, such as stainless steel or nitinol hypotubes, or may be fabricated by thin film vacuum deposition techniques. In accordance with the present invention, it is preferable to fabricate the inventive implantable materials and resulting devices by vacuum deposition of either or both of the base implant material and the chemically and/or physiochemically active features. Vacuum deposition permits greater control over many material characteristics and properties of the resulting material and formed device. For example, vacuum deposition permits control over grain size, grain phase, grain material composition, bulk material composition, surface topography, mechanical properties, such as transition temperatures in the case of a shape memory alloy. Moreover, vacuum deposition processes will permit creation of devices with greater material purity without the introduction of large quantities of contaminants that adversely affect the material and, therefore, the mechanical and/or biological properties of the implanted device. Vacuum deposition techniques also lend themselves to fabrication of more complex devices than those that are manufactured by conventional cold-working techniques. For example, multi-layer structures, complex geometrical configurations, extremely fine control over material tolerances, such as thickness or surface uniformity, are all advantages of vacuum deposition processing.
In vacuum deposition technologies, materials are formed directly in the desired geometry, e.g., planar, tubular, etc. The common principle of vacuum deposition processes is to take a material in a minimally processed form, such as pellets or thick foils, known as the source material and atomize them. Atomization may be carried out using heat, as is the case in physical vapor deposition, or using the effect of collisional processes, as in the case of sputter deposition, for example. In some forms of deposition a process such as laser ablation, which creates microparticles that typically consist of one or more atoms, may replace atomization; the number of atoms per particle may be in the thousands or more. The atoms or particles of the source material are then deposited on a substrate or mandrel to directly form the desired object. In other deposition methodologies, chemical reactions between ambient gas introduced into the vacuum chamber, i.e., the gas source, and the deposited atoms and/or particles are part of the deposition process. The deposited material includes compound species that are formed due to the reaction of the solid source and the gas source, such as in the case of chemical vapor deposition. In most cases, the deposited material is then either partially or completely removed from the substrate, to form the desired product.
A first advantage of vacuum deposition processing is that vacuum deposition of the metallic and/or pseudometallic films permits tight process control and films may be deposited that have a regular, homogeneous atomic and molecular pattern of distribution along their fluid-contacting surfaces. This avoids the marked variations in surface composition, creating predictable oxidation and organic adsorption patterns and has predictable interactions with water, electrolytes, proteins and cells. In particular, EC migration is supported by a homogeneous distribution of binding domains that serve as natural or implanted cell attachment sites in order to promote unimpeded migration and attachment.
Secondly, in addition to materials and devices that are made of a single metal or metal alloy layer, the inventive grafts may be comprised of a layer of biocompatible material or of a plurality of layers of biocompatible materials formed upon one another into a self-supporting multilayer structure because multilayer structures are generally known to increase the mechanical strength of sheet materials, or to provide special qualities by including layers that have special properties such as superelasticity, shape memory, radio-opacity, corrosion resistance etc. A special advantage of vacuum deposition technologies is that it is possible to deposit layered materials and thus films possessing exceptional qualities may be produced (cf., H. Holleck, V. Schier: Multilayer PVD coatings for wear protection, Surface and Coatings Technology, Vol. 76-77 (1995) pp. 328-336). Layered materials, such as superstructures or multilayers, are commonly deposited to take advantage of some chemical, electronic, or optical property of the material as a coating; a common example is an antireflective coating on an optical lens. Multilayers are also used in the field of thin film fabrication to increase the mechanical properties of the thin film, specifically hardness and toughness.
Thirdly, the design possibilities for possible configurations and applications of the inventive graft are greatly realized by employing vacuum deposition technologies. Specifically, vacuum deposition is an additive technique that lends itself toward fabrication of substantially uniformly thin materials with potentially complex three dimensional geometries and structures that cannot be cost-effectively achieved, or in some cases achieved at all, by employing conventional wrought fabrication techniques. Conventional wrought metal fabrication techniques may entail smelting, hot working, cold working, heat treatment, high temperature annealing, precipitation annealing, grinding, ablation, wet etching, dry etching, cutting and welding. All of these processing steps have disadvantages including contamination, material property degradation, ultimate achievable configurations, dimensions and tolerances, biocompatibility and cost. For example conventional wrought processes are not suitable for fabricating tubes having diameters greater than about 20 mm, nor are such processes suitable for fabricating materials having wall thicknesses down to about 1 μm with sub-μm tolerances.
Overall rate to reach confluence for the endothelial cells on the blood contact surface of implanted medical device is mainly determined by two factors, the rate of cell movement and rate of cell proliferation, with the first being more important. The rate of cell movement further comprises three interrelated steps. Initially, a cell forms lamellipodia and flopodia that protrude outward. This step involves reassembly of actins in the forefront of lambaepolia. After protrusion of lamellipodia from one or multiple points from the cell membrane, the front end of the lamellipodia will form a close attachment, called focal adhesion point, to the substratum through the interaction of integrin on the cell membrane and extrcellular matrix binding site. The final step of cell movement involves the contraction of the posterior end through the action of myosin II. The formation of a focal adhesion point is critical for the cell movement because the protruding lamellipodia will otherwise fold back. Without the tension force from the focal adhesion point, a cell loses the contraction from the posterior end and hence stops moving.
Availability of attachment sites on the substratum is not only important for the focal adhesion point formation, but also important for propagation. It has been shown that cells are forced to spread, survive better and proliferate faster than cells that are confined to the same amount of surface area (Science 276:1425-1428, 1997). This may explain why spreading of neighbor cells stimulate a cell to proliferate, after cells are lost from epithelium.
The formation of extracellular matrix (ECM) is, to much extent, determined by the cells within it because molecules which form ECM are secreted by the cells. Subsequently, the structure of the ECM, and hence the distribution of attachment sites on the ECM for the integrin binding, determines the focal adhesion point formation, the critical step in cell movement. Therefore, proper distribution of integrin binding sites on the surface of an implanted medical device substantially determines the speed of re-endothelialization from the ends surrounding the device.
There still remains a need for a medical device that stimulates endothelial proliferation and movement when implanted in order to form an endothelial layer over the medical device. Furthermore, there is a remaining need for a method of fabricating such a medical device.